1. Field of the Invention
The present invention relates to a method for manufacturing a positive electrode active material for a power storage device.
2. Description of the Related Art
Lithium-ion secondary batteries, which are small, lightweight, and reliable, have been widely used as power supplies of portable electronic devices. In addition, development of an electrically propelled vehicle on which a lithium-ion secondary battery is mounted has also progressed rapidly owing to growing awareness of environmental problems and energy problems.
As a positive electrode active material of a lithium-ion secondary battery, a phosphate compound having an olivine structure, such as LiFePO4, LiMnPO4, LiCoPO4, or LiNiPO4, has been known. However, the phosphate compound has a problem in that the charging capacity is limited and the operation voltage is high due to its structure. Thus, it is proposed to use a silicate-based (silicate) compound, such as LiFeSiO4 or LiMnSiO4 as a positive electrode active material due to the same olivine structure and high theoretical charging capacity.
As a synthesis method of a silicate-based (silicate) compound, such as LiFeSiO4 or LiMnSiO4, which can be used as a positive electrode active material of a lithium-ion secondary battery, a hydrothermal synthesis method and a solid phase reaction method have been known. In a hydrothermal synthesis method, microparticulation of a compound can be achieved, whereas in a solid phase reaction method, the mass production is possible and cost reduction can be achieved, which is preferable.
However, in the case of a solid phase reaction method, a mixture material in which materials of a positive electrode active material are mixed normally needs to be processed for a long time at a high temperature so that the reactivity is enhanced. As a result, the crystal grain size of the synthesized compound becomes large, which causes a significant problems for a positive electrode active material, such as a reduction in electron conductivity and a deterioration in capacitance characteristics, and therefore various researches have been made.